Electro coating is a process in which a metal substrate is submerged in a paint bath, and a direct current electric potential is used to cause paint to be deposited onto the metal substrate. The paint can be classified as cathodic or anodic, with cathodic being when the substrate being painted is the cathode and anodic being when the substrate is the anode. The other electrode, either the anode or cathode, is also submerged in the paint bath. The paint is water soluble but poorly ionized and therefore must have chemicals added to increase the ionization. In cathodic paint, the additives are weak acids, and in anodic paint they are weak bases. As the paint is deposited onto the substrate, the additives are freed from the paint and begin to build up in the paint bath. After a short time, the additives will prevent the further deposition of paint, because the pH will either increase (anodic paint) or decrease (cathodic paint) until paint is dissolved off of the metal substrate. To combat this problem, membrane electrode cells have been used for many years. One such cell is described in U.S. Pat. No. 4,711,709.
An electrode cell includes an ion exchange membrane, which is mounted to a structural component to form a water tight chamber within the paint bath. An electrode is installed inside the chamber, along with water and additives. Since the ion exchange membrane allows ions to pass through, and since the ions are drawn into the water tight chamber by the DC potential, the pH of the paint bath can be controlled by controlling the pH inside the water tight chamber.
While the ion exchange membrane allows ions and the electric potential to pass into the paint, it keeps the paint outside the water tight chamber and keeps the water inside. The water tight chamber is continually flushed with a solution of water and additives by means of an external piping system. The fluid that leaves the water tight chamber goes into a tank, where the pH is controlled by purging and adding fresh water. Then, water from the tank goes back into the water tight chamber in order to maintain the proper pH in the water tight chamber and in the paint bath.
These electrode cells generally come in four types: flat or box type, semicircular, curved or low profile, and round. They are available in many sizes of each type, both in length and in electrode width or diameter. All types have electrodes that can easily be removed without removing the cell from the bath. In the case of cathodic paint, which is most common, the electrode is an anode which wears over time and must be replaced on a regular basis. The length of time an anode will last depends on many factors, including pH in the cell, current load, the presence of contaminants in the cell such as bacteria, chlorides, or iron oxide from the anode itself. The membranes lose efficiency over time and must be replaced periodically as well. The same factors that affect the life of the electrode also affect the life of the membrane.
One problem that shortens the life of the electrode and the membrane is the presence of heavy particles of bacteria and iron oxide, which tend to build up in the cells. In the case of cathodic paint, these particles cause localized wear on the bottom of the anode and, with both cathodic and anodic paint, they cause deterioration of membrane efficiency.
In the prior art cells, the only way to replace the membranes is to remove the cell from the bath and disassemble it to replace the membrane or discard the cell entirely and replace it with an entirely new cell. Existing cells use a system in which the membrane is held in place by bolts, which pass through clamping bars or frames, through a plastic mesh, and through the membrane, a gasket, and fiber-reinforced plastic or plastic back. The process of disassembly and reassembly can take several hours, depending upon the skill of the workers and the type of cell. In large paint systems, such as automotive plants, this means hundreds of man hours per year.